Outbound transformation replaces both source and destination address with
state's end-point addresses at the same time when IPsec tunnel mode.
It is also required to change them for Mobile IPv6 route optimization, but we
should care about the following differences:
- changing result is not end-point but care-of address
- either source or destination is replaced for each state
This hook is a common platform to change outbound address.
Based on MIPL2 kernel patch.